C. Matter and Energy · Pure Substances and Mixtures

Pure Substances and Mixtures

C2.2 A1.2 A1.5About 40 minutes

Learning goals

I can explain the difference between a pure substance and a mixture using the particle theory.
I can draw particle diagrams of a pure substance and a mixture.
I can classify everyday materials as pure substances or mixtures and explain my thinking.

Words to know

pure substance
Matter made of only one kind of particle. Every sample of it has the same properties.

mixture
Matter made of two or more different kinds of particles (two or more pure substances) mixed together.

particle diagram
A drawing that uses circles or other shapes to show the particles in a sample of matter.

property
A feature that describes a substance, such as colour, melting point or whether it dissolves in water.

melting point
The temperature at which a solid melts. A pure substance melts at one exact temperature.

What's in your glass?

Pour a glass of tap water, a glass of apple juice and a glass of milk. They all look like simple liquids. But only some liquids are made of just one substance. Most of what you eat, drink and breathe is actually a mixture of several substances.

In this lesson you will use the particle theory to tell the two apart.

Three glasses on a table holding clear water, golden apple juice and white milk. — These three liquids look simple, but apple juice and milk are mixtures of many substances. Even tap water has small amounts of dissolved minerals. AI-generated image

Two big groups of matter

Remember postulate 2: each pure substance has its own kind of particle. That gives us a simple way to sort all matter.

A pure substance is made of only one kind of particle. Examples: pure water (distilled water), sugar, baking soda, pure gold, the helium in a party balloon.
A mixture contains two or more kinds of particles, because it is made of two or more pure substances. Examples: tap water, apple juice, air, soil, a salad, a granola bar.

The word pure in science has a special meaning. Spring water advertised as "pure" is still a mixture, because it has minerals dissolved in it. In science, pure means one kind of particle only.

Particle diagrams. Left: a pure substance has only one kind of particle (all the same). Right: a mixture has two kinds of particles mixed together.

Clues that something is a pure substance

You can't see particles, so scientists use properties as clues.

A pure substance has fixed properties. Pure water always freezes at 0 °C and boils at 100 °C (at sea level). Every sample of pure water is exactly the same.
A mixture's properties can change depending on how much of each part it has. Salt water freezes below 0 °C, and the more salt, the lower the freezing point. That's why salt is spread on Canadian roads in winter.
The parts of a mixture can often be separated by physical means, like filtering or evaporating. Evaporate salt water and the salt is left behind.

Material	Pure substance or mixture?	Why
Distilled water	Pure substance	Only water particles
Tap water	Mixture	Water plus small amounts of dissolved minerals and chlorine
Table sugar	Pure substance	Only sugar (sucrose) particles
Air	Mixture	Mostly nitrogen and oxygen, plus other gases
24-karat gold	Pure substance	Only gold particles
14-karat gold jewellery	Mixture	Gold mixed with other metals, such as silver or copper
Maple syrup	Mixture	Water, sugars and small amounts of minerals
Baking soda	Pure substance	Only one substance, sodium bicarbonate

Sorting everyday materials.

Worked example

Let's sort and draw: tap water or distilled water?

Classify distilled water and tap water, and draw a particle diagram of each.

Distilled water is made by boiling water and collecting the steam that condenses. The dissolved minerals are left behind. So it has only water particles: a pure substance.
Tap water comes from a lake, river or well. It has tiny amounts of dissolved minerals, and Canadian cities add a little chlorine to kill germs. So it has more than one kind of particle: a mixture.
Draw it: for distilled water, draw a box filled with identical circles. For tap water, draw mostly the same circles plus a few different shapes (triangles or a different shading) for the minerals and chlorine.
Check: does each drawing match the definition? Pure = one kind of particle. Mixture = two or more kinds.

Distilled water is a pure substance (one kind of particle). Tap water is a mixture (water particles plus a few particles of other substances).

The worked example as particle diagrams, with a key. Distilled water has only one kind of particle. Tap water is mostly water particles plus a few particles of other substances.

Draw a key

Every particle diagram needs a key that says what each shape stands for, for example ● = water particle, ▲ = salt particle. Keep the number of particles small (10 to 20) so the diagram is easy to read.

Just for fun

Funny but true: in science class, a bottle of "pure spring water" is a mixture, because it has minerals dissolved in it. Calling it pure is a bit like calling a fruit salad "pure banana".

Hands-on activity

Kitchen sort

You need: 10 food or household items from your kitchen (for example: salt, sugar, baking soda, flour, milk, orange juice, vinegar, honey, peanut butter, a bottle of water), paper and a pencil.

Don't open or taste anything you're not sure is food. Only look at labels and the items. Leave cleaning products in the cupboard.

Make a two-column table: Pure substance and Mixture.
For each item, read the ingredients list on the label. If it lists more than one ingredient, it's a mixture!
Sort each item and write one reason.
Choose one pure substance and one mixture. Draw a particle diagram of each, with a key.
Share it: explain your sort to a family member using the word particle.

What to look for: salt (if the label lists only salt, with no iodide or anti-caking agent), sugar and baking soda are pure substances. Flour, milk, juice, vinegar, honey and peanut butter are mixtures. Bottled water is a mixture too (it has dissolved minerals) unless it is labelled distilled. Note: iodized table salt is a mixture, because iodine compounds are added to the salt. Check your salt label!

Your turn

What is a pure substance?

What is a mixture?

Is air a pure substance or a mixture? Why?

Is a block of pure aluminium a pure substance or a mixture?

Why is tap water a mixture but distilled water is not?

Describe a particle diagram of a mixture of two substances.

Why does salt water freeze below 0 °C, while pure water freezes at exactly 0 °C?

More practice: warm-up

Pure substance or mixture? Sugar.

Pure substance or mixture? Orange juice.

Pure substance or mixture? A bowl of cereal with milk.

Pure substance or mixture? Helium in a balloon.

Pure substance or mixture? Soil.

More practice: on your own

A label on a bottle says "100% pure spring water. Contains calcium and magnesium." Is it a pure substance in the science sense? Explain.

Is 14-karat gold a pure substance? Explain.

A sample of an unknown clear liquid boils at exactly 100 °C, and the temperature stays at 100 °C until all of it is gone. Is it more likely pure or a mixture? Why?

A student draws 10 identical circles in a box and labels it "salad". What is wrong?

Why are road crews in Canada able to use salt to melt ice in winter?

Challenge

Maple syrup and sugar water can both be sweet, sticky liquids. Explain how you could tell, without tasting, that both are mixtures and not pure substances.

Leo says, "If it looks the same all the way through, it's a pure substance." Give a counter-example and explain.

Draw (or describe) particle diagrams for: (a) oxygen gas, (b) air. Explain how they differ.

Check your understanding

Quiz

A pure substance is made of...

Which of these is a mixture?

Which of these is a pure substance?

A particle diagram shows two different shapes mixed together. It shows...

Why is tap water called a mixture?

In science, the word pure means...

For parents and teachers

Big idea: Ontario expectation C2.2 asks students to use the particle theory to distinguish between pure substances (one kind of particle) and mixtures (more than one kind). The kitchen sort uses labels as evidence (A1.2) and asks your child to explain their reasoning (A1.5).

How to help: Read ingredient lists together. More than one ingredient means a mixture. Ask your child to draw quick particle diagrams with a key.

Common difficulties:

Using the everyday meaning of "pure". "Pure" orange juice and "pure" spring water are still mixtures in science.
Thinking anything that looks uniform is pure. Air, tap water and apple juice look the same throughout but are mixtures.
Iodized salt: it is a mixture, because iodine compounds are added. Plain salt with nothing else on the ingredients list is a pure substance.

Safety: sorting is done by looking at items and labels. Keep cleaning products closed and out of the activity.